This invention relates to vehicle mounted hook lift hoists and in particular to mechanisms for operatively coupling a tilt frame of the hoist to the frame of the truck.
Hook lift hoists are used to load and unload relatively large containers, bins, pallets, flat racks and the like onto a truck. The hoist generally includes an L-shaped boom or lift arm pivotally connected relative to the truck frame at a rear end of a first leg of the boom and a hook formed on the distal end of the second leg of the boom. Hydraulic actuators are used to pivot the L-shaped boom to position the hooked end behind the truck and to engage a container or the like. The hydraulic actuators are then retracted to pivot the hooked end of the boom toward the front of the truck pulling the container onto the truck bed. The process is reversed to unload the container from the truck. Rollers mounted on at least the rear of the truck facilitate loading and unloading the container therefrom. The first leg of the boom may be formed from telescoping members to permit the second leg to be extended away from the rear of the truck to facilitate lifting and pulling containers onto the truck frame.
As shown in U.S. Pat. No. 5,290,138, it is also known to pivotally mount the L-shaped boom or lift arm on a tilt frame that is in turn pivotally connected to the truck frame to permit the contents of a container to be dumped while supported on the tilt frame. In FIG. 4D of U.S. Pat. No. 5,290,138, the tilt frame is shown pivoted relative to the truck frame to assist in lowering the hook to a relatively low position. It is also known to latch the tilt frame to the truck frame near the front of the tilt frame when the L-arm is in its retracted position and used to load and unload a container to prevent the tilt frame from unintentionally raising when loading and unloading a container.
If the operator of the hook lift hoist, in attempting to lift a container off of the ground, inadvertently extends instead of retracts the hydraulic actuators controlling movement of the lift arm, a significant upward force can be exerted on the tilt frame where it is connected to the lift arm. The upward forces exerted on the tilt frame due to such an error can bend or otherwise damage the tilt frame members. To avoid such bending or damage, it is known to increase the strength of the tilt frame by forming it out of larger, thicker or heavier material or beams. However, doing so adds to the cost and the weight of the vehicle mounted hook lift hoist and therefore is an undesirable solution.